Switch mode power converters (SMPC) that convert AC or DC input power into DC output(s) frequently have multiple outputs. These outputs are often derived from multiple secondary windings of a single power transformer. Alternatively, multiple power transformers may also be used. In SMPCs having a single power transformer, a primary winding of the power transformer is switched or commutated to an input voltage source by converter switches in such a way as to provide pulse trains at appropriate current and voltage levels on the secondary windings. The DC outputs are formed via rectification and subsequent filtering of the pulse trains. A voltage level of each DC output, therefore, depends on a turns ratio of each secondary winding to the primary winding as well as a duty cycle of the converter switches.
One DC output (usually called a "main output") may be directly regulated by a feedback circuit. The feedback circuit senses the main output and modifies a switching pattern of the converter switches to compensate for changes in either load or input voltage thereby maintaining the main output at a relatively constant voltage level. Many methods of direct feedback regulation are known in the art, including, for instance, pulse width modulation (PWM).
While direct feedback regulation is adequate for regulating the main output, the other DC outputs (auxiliary outputs) cannot be similarly regulated. Although direct feedback regulation of the main output provides some measure of regulation (cross-regulation) to the auxiliary outputs due to transformer coupling, in many cases the cross-regulation is inadequate. One technique, post-regulation, has been developed to allow regulation of the auxiliary outputs.
A conventional post regulator typically includes a rectifier, coupled to a secondary winding, that rectifies the pulse train generated by the primary side converter switches. A switch, series-coupled to the rectifier, then uses leading-edge modulation to regulate a pulse width of the pulse train. By blocking a portion of the pulse train, the switch may thus regulate a voltage level of the corresponding auxiliary output.
While leading-edge modulation provides adequate regulation of the auxiliary output, trailing-edge modulation is preferred. Trailing-edge modulation allows conventional low cost PWM devices to be used. Additionally, current-mode control, which enhances transient response and allows cycle-by-cycle current limitation, is possible with trailing-edge modulation. To achieve high efficiency operation with trailing-edge modulation, however, leakage energy from the secondary winding of the transformer must be recovered. Reverse-recovery energy of the rectifier must also be recovered to reduce power loss and noise emissions.
Accordingly, what is needed in the art is a system and method for recovering energy from the post regulator, thereby increasing an overall efficiency of the power converter.